1. Field of the Invention
The invention relates generally to blast focusing methods and apparatus, and, more particularly, to blast focusing methods and apparatus for terrain clearance, especially clearance of mine fields containing hard-to-kill land mines.
2. Description of the Prior Art
Explosive devices are normally initiated by one detonator, also called a blasting cap, which is sometimes augmented by a booster. The design of detonators and explosives is aimed at very dependable, non-accidental firing. There is nothing special about standard explosives in regard to tailoring their destructiveness. Blast destructiveness depends on the explosive's mass and distance to the target. With the explosive's energy released as a shock wave in all directions, mostly non-target directions, much of the energy is wasted. Thus, the explosive must land fairly close to the target.
Some explosive devices, generally called blast focusing warheads, include various means to redistribute an explosives mass's energy. The object of most blast focusing devices is to direct the blast in order to propel fragments to extra-high velocity at a target. Two well known examples of blast focusing devices are the spherical or conical shaped charge and the linear shaped charge, which are designed respectively for metal penetration and metal cutting. The focusing action is due to the explosive shape and the standoff distance.
U.S. Pat. No. 3,136,251 issued June 9, 1964 to Witow, describes a directional warhead in which the blast is directed by plates within the explosive that are charged negatively in the direction of the target as sensed by a proximity device. The explosive casing is positively charged. When detonated, positive ions move to the internal plate while free electrons move to the inner surface of the case and act as a shock absorber. For a instant of time, the case will fragment first and blast flow is increased in the negative plate direction.
U.S. Pat. No. 3,598,051, issued Aug. 10, 1971 to Avery, discloses a spherical charge with twelve symmetrically placed detonators that are fired in pairs, some at once and others equally delayed in order to create detonation waves that would collide constructively along one of nine possible firing axes. Electronic sensing and switching will fire the warhead along the axis pointing at the desired target.
In another known directional warhead, the blast is focused by explosively indenting a cased explosive. An instant later, the main charge is initiated and the blast favors the indentation location, which is electronically set in a target's direction.
In Lab Memos MM-2-092, dated June 17, 1966, and MM-2-110, dated Mar. 27, 1967, of the Applied Physics Laboratory, John Hopkins' University, tests were reported by Ferrenz, Donnelly, and Robertson on a focused blast warhead in which the explosive charge had an unusual spool shape, with a hollow core, that was detonated simultaneously at each end through wave shaping devices.
U.S. Pat. No. 4,273,048 issued June 16, 1981 to Aley et al, describes a fuel-air explosive weapon launched from a remote distance for clearing minefields. During flight, a parachute at the back of the round is deployed to slow the round and align it vertically. Thereafter, a compacted probe at the front of the round is extended. When this probe makes contact with the ground, it initiates deployment of two firing cloud detonators and thereafter initiates a burster charge which disperses a fuel-air cloud over a section of the minefield. Detonation of this cloud by the cloud detonators produces high pressure under the cloud which causes triggering of the mines.
Some hard-to-kill land mines can only be explosively detonated by very high blast pressure. While fuel-air explosive warheads produce high pressure over a large ground area when detonated, these warheads generally do not produce the very high peak pressures required to detonate these hard-to-kill mines. These very high pressures can be produced by unfocused solid explosive warheads; however, the area of the ground plane subject to these very high pressures is low relative to the quantity of solid explosive detonated.